Overexpression Of TRAF6 Research Articles

Tumor Necrosis Factor Receptor–Associated Factor 2 Signaling Provokes Adverse Cardiac Remodeling in the Adult Mammalian Heart

Tumor necrosis factor superfamily ligands provoke a dilated cardiac phenotype signal through a common scaffolding protein termed tumor necrosis factor receptor-associated factor 2 (TRAF2); however, virtually nothing is known about TRAF2 signaling in the adult mammalian heart. We generated multiple founder lines of mice with cardiac-restricted overexpression of TRAF2 and characterized the phenotype of mice with higher expression levels of TRAF2 (myosin heavy chain [MHC]-TRAF2(HC)). MHC-TRAF2(HC) transgenic mice developed a time-dependent increase in cardiac hypertrophy, left ventricular dilation, and adverse left ventricular remodeling, and a significant decrease in LV+dP/dt and LV-dP/dt when compared with littermate controls (P<0.05 compared with littermate). During the early phases of left ventricular remodeling, there was a significant increase in total matrix metalloproteinase activity that corresponded with a decrease in total myocardial fibrillar collagen content. As the MHC-TRAF2(HC) mice aged, there was a significant decrease in total matrix metalloproteinase activity accompanied by an increase in total fibrillar collagen content and an increase in myocardial tissue inhibitor of metalloproteinase-1 levels. There was a significant increase in nuclear factor-κB activation at 4 to 12 weeks and jun N-terminal kinases activation at 4 weeks in the MHC-TRAF2(HC) mice. Transciptional profiling revealed that >95% of the hypertrophic/dilated cardiomyopathy-related genes that were significantly upregulated genes in the MHC-TRAF2(HC) hearts contained κB elements in their promoters. These results show for the first time that targeted overexpression of TRAF2 is sufficient to mediate adverse cardiac remodeling in the heart.

TRAIL-induced expression of uPA and IL-8 strongly enhanced by overexpression of TRAF2 and Bcl-xL in pancreatic ductal adenocarcinoma cells

The death ligand, tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), induces apoptosis and non-apoptotic signaling in some tumor cells. The purpose of this study was to investigate the roles of the pro-apoptotic TRAIL receptors, TRAIL-R1 and TRAIL-R2, as well as Bcl-xL and TRAF2 in TRAIL-induced expression of the pro-inflammatory cytokine IL-8 and the invasion-promoting protein urokinase (uPA) in pancreatic ductal adenocarcinoma (PDAC) cells. Colo357wt, Colo357/TRAF2, Colo357/Bcl-xL, Panc89 and PancTuI cells were stimulated with TRAIL and uPA and IL-8 expression was detected using real-time PCR. Antagonistic, receptor-specific antibodies were used to investigate the effects of TRAIL-R1 or TRAIL-R2 inhibition. Dose-dependent increases in uPA and IL-8 expression were detected following TRAIL stimulation in PDAC cells. These effects were inhibited when TRAIL-R1 but not TRAIL-R2 was blocked. Overexpression of TRAF2 or Bcl-xL strongly increased TRAIL-mediated upregulation of uPA and IL-8. In PDAC cells, TRAIL strongly induced uPA and IL-8 via TRAIL-R1. This response was further enhanced in cells overexpressing TRAF2 and Bcl-xL. Therefore, inhibition of the non-apoptotic "side-effects" of TRAIL treatments by inactivation of TRAF2 and Bcl-xL might represent additional relevant strategies for the treatment of pancreatic cancer.

TRAF6 regulates proliferation, apoptosis, and invasion of osteosarcoma cell

TRAF6, a unique tumor necrosis factor receptor-associated factor (TRAF) family member, possesses a unique receptor-binding specificity that results in its crucial role as the signaling mediator for TNF receptor superfamily and interleukin-1 receptor/Toll-like receptor superfamily. TRAF6 plays an important role in tumorigenesis, invasion and metastasis. This study aimed to explore the expression of TRAF6 in osteosarcoma tissues and its correlation to the clinical pathology of osteosarcoma and to discuss the relationship between TRAF6 expression and osteosarcoma invasion. These data will provide the experimental base for the biological treatment of osteosarcoma in the future. Using RT-PCR and Western blot, the results showed that the expression rate of TRAF6 mRNA in osteosarcoma tissues was significantly higher than that in normal bone tissue (p < 0.05), that the expression rate of TRAF6 mRNA in the carcinoma tissues from patients with lung metastasis was significantly higher than that from patients without lung metastasis (p < 0.05), and that the expression rate of TRAF6 mRNA also increased with increasing Enneking stage (p < 0.05). However, the mRNA expression of TRAF6 in osteosarcoma was independent of the patient's gender, age, and tumor size (p > 0.05). The TRAF6 protein displayed an up-regulation in osteosarcoma tissues compared to normal bone tissue (p < 0.05), displayed an up-regulation in osteosarcoma tissues from patients with lung metastasis compared to from patients without lung metastasis (p < 0.05), and displayed a gradual increase with increasing Enneking stage (p < 0.05). By the technique of RNA interference, the expression of TRAF6 in the human osteosarcoma MG-63 cell line was down-regulated, and the invasive ability of MG-63 cells was examined. The results showed that TRAF6 protein expression was significantly decreased in the MG-63 cells from TRAF6 siRNA-transfected group (p < 0.05), and the proliferation ability of MG-63 cells and the number of MG-63 cells that passed through the Transwell chamber were significantly lower than that in the non-transfected control group as well as the transfected control group (p < 0.05). In addition, the percentage of MG-63 cells undergoing apoptosis was significantly higher in the TRAF6 siRNA-transfected group compared with the non-transfected control group as well as the transfected control group (p < 0.05). The expression of p-p65, cyclin D1, MMP-9 was down-regulated in the MG-63 cells from TRAF6 siRNA-transfected group. The expression of caspase 3 was up-regulated in the MG-63 cells from TRAF6 siRNA-transfected group compared to the non-transfected control group as well as the transfected control group (p < 0.05). To make a long story short, the overexpression of TRAF6 in osteosarcoma might be related to the tumorigenesis, invasion of osteosarcoma.

Hepatic TRAF2 Regulates Glucose Metabolism Through Enhancing Glucagon Responses

Obesity is associated with intrahepatic inflammation that promotes insulin resistance and type 2 diabetes. Tumor necrosis factor receptor–associated factor (TRAF)2 is a key adaptor molecule that is known to mediate proinflammatory cytokine signaling in immune cells; however, its metabolic function remains unclear. We examined the role of hepatic TRAF2 in the regulation of insulin sensitivity and glucose metabolism. TRAF2 was deleted specifically in hepatocytes using the Cre/loxP system. The mutant mice were fed a high-fat diet (HFD) to induce insulin resistance and hyperglycemia. Hepatic glucose production (HGP) was examined using pyruvate tolerance tests, 2H nuclear magnetic resonance spectroscopy, and in vitro HGP assays. The expression of gluconeogenic genes was measured by quantitative real-time PCR. Insulin sensitivity was analyzed using insulin tolerance tests and insulin-stimulated phosphorylation of insulin receptors and Akt. Glucagon action was examined using glucagon tolerance tests and glucagon-stimulated HGP, cAMP-responsive element–binding (CREB) phosphorylation, and expression of gluconeogenic genes in the liver and primary hepatocytes. Hepatocyte-specific TRAF2 knockout (HKO) mice exhibited normal body weight, blood glucose levels, and insulin sensitivity. Under HFD conditions, blood glucose levels were significantly lower (by >30%) in HKO than in control mice. Both insulin signaling and the hypoglycemic response to insulin were similar between HKO and control mice. In contrast, glucagon signaling and the hyperglycemic response to glucagon were severely impaired in HKO mice. In addition, TRAF2 overexpression significantly increased the ability of glucagon or a cAMP analog to stimulate CREB phosphorylation, gluconeogenic gene expression, and HGP in primary hepatocytes. These results suggest that the hepatic TRAF2 cell autonomously promotes hepatic gluconeogenesis by enhancing the hyperglycemic response to glucagon and other factors that increase cAMP levels, thus contributing to hyperglycemia in obesity.

SHARPIN Negatively Associates with TRAF2-Mediated NFκB Activation

NFκB is an inducible transcriptional factor controlled by two principal signaling cascades and plays pivotal roles in diverse physiological processes including inflammation, apoptosis, oncogenesis, immunity, and development. Activation of NFκB signaling was detected in skin of SHAPRIN-deficient mice and can be diminished by an NFκB inhibitor. However, in vitro studies demonstrated that SHARPIN activates NFκB signaling by forming a linear ubiquitin chain assembly complex with RNF31 (HOIP) and RBCK1 (HOIL1). The inconsistency between in vivo and in vitro findings about SHARPIN's function on NFκB activation could be partially due to SHARPIN's potential interactions with downstream molecules of NFκB pathway. In this study, 17 anti-flag immunoprecipitated proteins, including TRAF2, were identified by mass spectrum analysis among Sharpin-Flag transfected mouse fibroblasts, B lymphocytes, and BALB/c LN stroma 12 cells suggesting their interaction with SHARPIN. Interaction between SHARPIN and TRAF2 confirmed previous yeast two hybridization reports that SHARPIN was one TRAF2's partners. Furthermore, luciferase-based NFκB reporter assays demonstrated that SHARPIN negatively associates with NFκB activation, which can be partly compensated by over-expression of TRAF2. These data suggested that other than activating NFκB signaling by forming ubiquitin ligase complex with RNF31 and RBCK1, SHARPIN may also negatively associate with NFκB activation via interactions with other NFκB members, such as TRAF2.

Increases In Mirna-145 and Mirna-146a Expression In Patients with IPSS Lower-Risk Myelodysplastic Syndromes and Del(5q) Treated with Lenalidomide.

AbstractAbstract 3631 Introduction:The erythroid differentiation defect observed in 5q– syndrome has been attributed to the RPS14 gene located within the CDR of the long arm of chromosome 5. We have recently demonstrated that RPS14 expression increases during lenalidomide treatment. However, haploinsufficiency of RPS14, which encodes ribosomal protein S14, does not explain clonal dominance. The expression of miRNAs, miR-145 (5q33.1) and miR-146a (5q33.3), in CD34+ bone marrow (BM) cells of individuals with MDS with deletion of the long arm of chromosome 5 (del(5q)) is lower compared to normal controls (Starczynowski et al, Nature Medicine, 2010). miRNAs are small noncoding RNAs that post-transcriptionally repress specific messenger RNA targets through interaction with the 3′ untranslated region (UTR). Loss of noncoding transcripts encoding miRNAs within the CDR may result in haploinsufficiency by loss of inhibition of their targets. Concurrent loss of both miR-145 and miR-146a resulted in activation of innate immune signalling through elevated expression of their respective targets, TIRAP and TRAF6. Furthermore, knockdown of miR-145 and miR-146a or overexpression of TRAF6 in mouse HSPC (Hematopoietic stem and progenitor cells) recapitulated features of 5q– syndrome, such as bone marrow dysplasia, anemia and thrombocytosis. We present preliminary results of changes in miRNA expression in IPSS lower-risk MDS with del(5q) during treatment with lenalidomide. Methods:A prospective single-arm trial investigating the efficacy and safety of lenalidomide in 46 patients with MDS with del(5q) with/without additional cytogenetic abnormalities and Hb < 10 g/dL. Lenalidomide was administered orally at a starting dose of 10 mg/day for a maximum of 12 months. When necessary, dosing was reduced to 5 mg/day or 5 mg on alternate days. Bone marrow assessments were performed at baseline and every 3 months, thereafter. For the evaluation of miRNA-145 and miRNA-146a in patient samples, 300 ng/μl of miRNAs were isolated in each purified BM sample by using mirVana™ miRNA Isolation Kit-Ambion and TaqMan miRNA Array Analysis was performed to determine the expression of miRNAs (7900HT Sequence Detection System Applied Biosystems). Patient BM-miRNAs were calibrated with miRNAs from BM of healthy volunteer donors. It was assumed that BM expression value of each calibrator miRNA was 1 unit. RPS14 gene assays were performed using TaqMan® Low Density Array Fluidic card (TaqMan® Human Array, Applied Biosystems, Foster City, CA, USA) based on Applied Biosystems PRISM® 7900HT comparative ddCT method, according to the manufacturer's instructions. Target gene expression levels were measured in triplicate and normalized against the expression of the 18S housekeeping gene from a BM pool of normal, healthy subjects at all timepoints. Median relative gene expression values in MDS patients were compared to healthy subjects, set as a value of 1. Results:Four patients have been evaluated (1 M, 3 F; ages 65, 66, 73 and 76 years, respectively) at baseline and after 12 weeks. At baseline, 2 patients were RBC-transfusion dependent. One patient had one additional cytogenetic abnormality (+8 in 15% metaphases). All patients obtained an erythroid response by week 12: mean Hb values significantly increased from 8.4 ± 0.9 at baseline to 11.6 ± 0.9 g/dL (p=0.01). All patients obtained a cytogenetic response, 2 of which were complete. miRNA-145 and miRNA-146a expression were both low at baseline and significantly increased by week 12 (Table). Conclusions:Preliminary results confirm that, in IPSS lower-risk MDS with del(5q), miRNA-145 and miRNA-146a expression is low. Lenalidomide treatment is associated with erythroid responses and cytogenetic remissions concurrent with significant increases in miRNA-145 and miRNA-146a expression.Table 1miRNA 145 and 146a and RPS14 expression before and during lenalidomide treatment.CasesmiRNA 145miRNA 146aRPS14Baseline12 weeksBaseline12 weeksBaseline12 weeks10.0004.8630.9361.7000.3057.62520.8363.8470.0871.6540.7198.83530.9276.9470.0342.7800.7352.03340.0672.8300.0740.8970.6376.164Mean ± SD0.458 ± 0.4924.622 ± 1.7580.283 ± 0.4361.758 ± 0.7750.599 ± 0.2016.164 ± 2.963p-value0.0130.0490.035 Disclosures:Oliva:Celgene: Consultancy.

Tumor necrosis factor receptor associated factor 6 is not required for atherogenesis in mice and does not associate with atherosclerosis in humans.

BackgroundTumor necrosis factor receptor-associated factors (TRAFs) are important signaling molecules for a variety of pro-atherogenic cytokines including CD40L, TNF α, and IL1β. Several lines of evidence identified TRAF6 as a pro-inflammatory signaling molecule in vitro and we previously demonstrated overexpression of TRAF6 in human and Murine atherosclerotic plaques. This study investigated the role of TRAF6-deficiency in mice developing atherosclerosis, a chronic inflammatory disease.Methodology/Principal FindingsLethally irradiated low density lipoprotein receptor-deficient mice (TRAF6+/+/LDLR−/−) were reconstituted with TRAF6-deficient fetal liver cells (FLC) and consumed high cholesterol diet for 18 weeks to assess the relevance of TRAF6 in hematopoietic cells for atherogenesis. Additionally, TRAF6+/−/LDLR−/− mice received TRAF6-deficient FLC to gain insight into the role of TRAF6 deficiency in resident cells. Surprisingly, atherosclerotic lesion size did not differ between the three groups in both aortic roots and abdominal aortas. Similarly, no significant differences in plaque composition could be observed as assessed by immunohistochemistry for macrophages, lipids, smooth muscle cells, T-cells, and collagen. In accord, in a small clinical study TRAF6/GAPDH total blood RNA ratios did not differ between groups of patients with stable coronary heart disease (0.034±0.0021, N = 178), acute coronary heart disease (0.029±0.0027, N = 70), and those without coronary heart disease (0.032±0.0016, N = 77) as assessed by angiography.ConclusionOur study demonstrates that TRAF6 is not required for atherogenesis in mice and does not associate with clinical disease in humans. These data suggest that pro- and anti-inflammatory features of TRAF6 signaling outweigh each other in the context of atherosclerosis.

Cbp recruitment of Csk into lipid rafts is critical to c-Src kinase activity and bone resorption in osteoclasts

A tyrosine kinase, c-Src, that plays an indispensable role in ruffled border formation and bone resorption is constitutively active in osteoclasts. However, to date, the molecular mechanism underlying increased c-Src activity in osteoclasts is unknown. To address this, we first examined the expression levels and subcellular localization of Csk, a negative regulatory kinase for c-Src. We found that the expression level of Csk in osteoclasts was comparable with that of other tissues. However, in osteoclasts, Csk was hardly localized in lipid rafts, where c-Src is highly expressed. Interestingly, expression of Cbp, which recruits Csk into lipid rafts through physical interaction with Csk, was very low in osteoclasts compared with other tissues. To understand the importance of Cbp in osteoclasts, we introduced Cbp into osteoclasts using an adenovirus gene delivery system. Introduction of Cbp stimulated recruitment of Csk into lipid rafts and suppressed c-Src activity in a dose-dependent manner. Furthermore, introduction of Cbp markedly inhibited formation of actin rings and bone-resorbing activity in osteoclasts. In addition, treatment with RANKL and overexpression of TRAF6 or NFAT2 inhibited Cbp expression in the osteoclastogenic cell line RAW264.7 along with osteoclastic differentiation. NFAT2 overexpression also inhibited Cbp expression in spleen macrophages. Collectively, our results indicate that reduction in Cbp expression is responsible for maintaining high c-Src activity in osteoclasts. These findings contribute to an understanding of the unique regulatory system for c-Src in osteoclasts.

TNFR1 functions as a survival receptor in TNFR2-/- CD8 T cells (50.19)

Abstract TNF/TNFR superfamily members play central roles in host defense, inflammation, apoptosis, autoimmunity and organogenesis. Previous reports showed that TNFR2 regulates T cell activation by lowering the activation threshold and providing costimulatory signaling. Furthermore, activated TNFR2-/- CD8 cells are highly resistant to AICD. Here, we show that blocking of activated wild-type (wt) CD8 cells with anti-TNFR2 antibodies also renders these cells resistant to AICD. The resistance of activated TNFR2-/- CD8 cells to AICD correlates with the accumulation of TRAF2. Retroviral transfection studies showed that TRAF2 overexpression was partially effective in preventing AICD in activated wt CD8+ cells. Furthermore, neutralization of TNF-α inhibits the proliferation of anti-CD3-stimulated wt CD8 T cells and increases apoptotic death cells in activated TNFR2-/- cells. TNF-α blocking also reduced TRAF2 accumulation in activated TNFR2-/- CD8 cells, which correlates with their increase susceptibility to AICD. AICD-resistant TNFR2-/- CD8 cells expressed elevated levels of phosphorylated IκBα and neutralization of TNF-α blocked this increase. These results indicate that in activated TNFR2-/- CD8 cells, TNFR1 functions as a survival receptor by promoting TNF-α-mediated regulation of TRAF2 levels and the extent of phosphorylation of pro-survival signaling molecules such as IκBα. This work is supported by the Canadian Cancer Society.

CIAP1-dependent TRAF2 degradation regulates the differentiation of monocytes into macrophages and their response to CD40 ligand

AbstractPeripheral blood monocytes are plastic cells that migrate to tissues and differentiate into various cell types, including macrophages, dendritic cells, and osteoclasts. We have described the migration of cellular inhibitor of apoptosis protein 1 (cIAP1), a member of the IAP family of proteins, from the nucleus to the Golgi apparatus in monocytes undergoing differentiation into macrophages. Here we show that, once in the cytoplasm, cIAP1 is involved in the degradation of the adaptor protein tumor necrosis factor receptor–associated factor 2 (TRAF2) by the proteosomal machinery. Inhibition of cIAP1 prevents the decrease in TRAF2 expression that characterizes macrophage formation. We demonstrate that TRAF2 is initially required for macrophage differentiation as its silencing prevents Iκ-Bα degradation, nuclear factor-κB (NF-κB) p65 nuclear translocation, and the differentiation process. Then, we show that cIAP1-mediated degradation of TRAF2 allows the differentiation process to progress. This degradation is required for the macrophages to be fully functional as TRAF2 overexpression in differentiated cells decreases the c-Jun N-terminal kinase–mediated synthesis and the secretion of proinflammatory cytokines, such as interleukin-8 and monocyte chemoattractant protein 1 (MCP-1) in response to CD40 ligand. We conclude that TRAF2 expression and subsequent degradation are required for the differentiation of monocytes into fully functional macrophages.

CIAP1-Dependent TRAF2 Degradation Is Required for Macrophage Differentiation and Function

TRAF2 (Tumor necrosis factor (TNF) Receptor associated factor) is an adaptor molecule involved in TNF family membrane receptor signalling pathway. We demonstrate that TRAF2 is required for macrophage differentiation. Its silencing prevents Iκ-Bα degradation, NF-κB p65 nuclear translocation and the differentiation process. Then, TRAF2 is degraded by the proteasome system in the cytoplasm. Inhibition or silencing of the E3 ubiquitine ligase cIAP1 from the IAP (Inhibitor of apoptosis protein) family of proteins, prevents the differentiation-associated TRAF2-degradation. In the presence of proteasome inhibitor, TRAF2 colocalizes with cIAP1 in a golgi-associated compartment, suggesting that cIAP-1-mediated TRAF2 degradation occurred in this subcellular compartment. An overexpression of TRAF2 or silencing of cIAP1 prevents the outcome of differentiation as revealed by the expression of differentiation markers at the cell surface, and inhibits the secretion of pro-inflammatory cytokines such as interleukin-8 and MCP-1 by fully differentiated-macrophages in response to CD40 stimulation. Our data suggest that TRAF2 expression and its subsequent cIAP1-dependent degradation is required for the differentiation of monocytes into macrophages and allows the macrophages to be fully functional.

Smurf2 is a TRAF2 binding protein that triggers TNF-R2 ubiquitination and TNF-R2-induced JNK activation

TRAF2 plays a central role in TNF-induced signalling to NF-κB and JNK/p38 MAPK. To better understand the molecular mechanisms that mediate this dual function of TRAF2, we performed a yeast two-hybrid screening for TRAF2 interacting proteins using the Sos recruitment system. This resulted in the identification of the E3 ubiquitin ligase Smurf2 as a TRAF2 binding protein. TRAF2 overexpression was shown to trigger Smurf2 ubiquitination and the formation of a TNF-R2/Smurf2 complex. Smurf2 on its turn promoted TNF-R2 ubiquitination and the relocalization of TNF-R2 as well as TRAF2 to a detergent-insoluble cell fraction. This was associated with enhanced TNF-R2-induced JNK activation, whereas TNF-R2-induced NF-κB activation remained unaffected. These results suggest an important role for Smurf2 binding to TRAF2 in determining specific signalling outputs of TNF-R2.

JNK/c-Jun Signaling Mediates an Anti-Apoptotic Effect of RANKL in Osteoclasts

RANKL is known to be important not only for differentiation and activation of osteoclasts but also for their survival. Experimentally, apoptosis of osteoclasts is rapidly induced by the deprivation of RANKL. RANKL activates Elk-related tyrosine kinase (ERK), p38, c-Jun N-terminal kinase (JNK), and NF-kappaB pathways through TRAF6 in osteoclasts and the precursor cells. It has been shown that ERK is critical for regulation of osteoclast survival. However, an involvement of other RANKL signaling pathways such as JNK signaling in survival of osteoclasts has not been fully understood yet. Osteoclasts derived from primary mouse bone marrow cells by soluble RANKL (sRANKL) were treated with a JNK inhibitor, SP600125, or infected with adenovirus carrying dominant-negative (DN)-c-jun, DN-c-fos, mitogen-activated protein kinase kinase 1 (MEKK1), I-kappaBalpha mutant, or NF-kappaB components, p50 and p65. Osteoclasts were cultured with or without sRANKL, and apoptotic phenotype was determined by TUNEL assay, DAPI staining, and expression of cleaved caspase 3 followed by TRACP staining. Overexpression of TRAF6 activated JNK and NF-kappaB signaling pathways and clearly prevented osteoclasts from apoptosis caused by abrogation of sRANKL. An anti-apoptotic effect of RANKL/RANK/TRAF6 signaling on osteoclast was inhibited by JNK-specific inhibitor SP600125 and by overexpression of dominant-negative JNK1, c-jun, and c-fos. Also, overexpression of MEKK1 inhibited apoptosis of osteoclasts even in the absence of sRANKL along with activation of JNK/c-jun signaling. On the other hand, blockade of NF-kappaB signaling by I-kappaBalpha mutant or overexpression of NF-kappaB components showed a marginal effect on apoptosis of osteoclasts. An important role of RANKL-induced activation of MEKK1/JNK/c-jun signaling in the regulation of apoptosis in osteoclasts was shown. Our study suggests that c-fos plays a role as a partner of activator protein-1 factor, c-jun, during the regulation of apoptosis in osteoclasts.

Tyrosine Phosphorylation of MAL in TLR4 Signaling and Endotoxin Tolerance

TLR4 recognition of LPS triggers signalosome assembly among TLR4, sorting (e.g. MyD88‐adapter‐like, Mal) and signaling (e.g. MyD88) adapters and kinases, resulting in activation of transcription factors and production of inflammatory mediators. Here we examined whether Mal tyrosine phosphorylation regulates Mal‐TLR4 interactions and signaling. In contrast to wild‐type (WT) Mal, overexpression of tyrosine‐deficient Mal variants with mutated Y86, Y106, and Y159 tyrosine residues impaired Mal tyrosine phosphorylation, interactions with Bruton's tyrosine kinase (Btk), phosphorylation of p38, and NF‐κB activation. LPS triggered tyrosine phosphorylation of endogenous Mal and initiated Mal‐Btk interactions in 293/TLR4/MD‐2 cells and human monocytes that were suppressed in endotoxin‐tolerant cells. Compared to WT‐Mal, Y86A‐ and Y159A‐ Mal variants exhibited higher interactions with TLR4 and MyD88, while associations with IRAK‐2 and TRAF‐6 were not affected. Overexpression of Y86A, Y159A and Y106A Mal in 293/TLR4/MD‐2 cells exerted dominant‐negative effects on TLR4‐inducible p38 phosphorylation and NF‐κB reporter activation. In contrast, tyrosine‐deficient Mal species did not affect NF‐κB activation when signaling was initiated at the post‐receptor level by overexpression of MyD88, IRAK‐2 or TRAF‐6. Thus, tyrosine phosphorylation of Mal is required for TLR signaling, regulates Mal interactions with TLR4 and receptor signaling, and is inhibited in endotoxin tolerance.This work was supported by NIH grant AI‐059524 (to A.E.M.)

Tyrosine Phosphorylation of MyD88 Adapter-like (Mal) Is Critical for Signal Transduction and Blocked in Endotoxin Tolerance

Toll-like receptor 4 (TLR4) recognition of lipopolysaccharide triggers signalosome assembly among TLR4, sorting (e.g. MyD88 adapter-like (Mal)) and signaling (e.g. MyD88) adapters, initiating recruitment and activation of kinases, activation of transcription factors, and production of inflammatory mediators. In this study we examined whether tyrosine phosphorylation of Mal regulates its interactions with TLR4, MyD88, interleukin-1 (IL-1) receptor-associated kinase (IRAK)-2, and tumor necrosis factor receptor-associated factor (TRAF)-6 and is important for signaling. Overexpression of wild-type Mal in human embryonic kidney 293T cells induced its constitutive tyrosine phosphorylation and led to activation of p38, NF-kappaB, and IL-8 gene expression. Mutagenesis of Tyr-86, Tyr-106, and Tyr-159 residues within the Toll-IL-1 receptor domain impaired Mal tyrosine phosphorylation, interactions with Bruton tyrosine kinase, phosphorylation of p38, and NF-kappaB activation. Lipopolysaccharide triggered tyrosine phosphorylation of endogenous Mal and initiated Mal-Bruton-tyrosine kinase interactions in 293/TLR4/MD-2 cells and human monocytes that were suppressed in endotoxin-tolerant cells. Compared with wild-type Mal, Y86A-, Y06A-, and Y159A-Mal variants exhibited higher interactions with TLR4 and MyD88, whereas associations with IRAK-2 and TRAF-6 were not affected. Overexpression of Y86A- and Y106A-Mal in 293/TLR4/MD-2 cells exerted dominant-negative effects on TLR4-inducible p38 phosphorylation and NF-kappaB reporter activation to the extent comparable with P125H-Mal-mediated suppression. In contrast, tyrosine-deficient Mal species did not affect NF-kappaB activation when signaling was initiated at the post-receptor level by overexpression of MyD88, IRAK-2, or TRAF-6. Thus, tyrosine phosphorylation of Mal is required for adapter signaling, regulates Mal interactions with TLR4 and receptor signaling, and is inhibited in endotoxin tolerance.

HTLV-1 Tax-mediated TAK1 activation involves TAB2 adapter protein

Human T cell leukemia virus type 1 (HTLV-1) Tax is an oncoprotein that plays a crucial role in the proliferation and transformation of HTLV-1-infected T lymphocytes. It has recently been reported that Tax activates a MAPKKK family, TAK1. However, the molecular mechanism of Tax-mediated TAK1 activation is not well understood. In this report, we investigated the role of TAK1-binding protein 2 (TAB2) in Tax-mediated TAK1 activation. We found that TAB2 physically interacts with Tax and augments Tax-induced NF-κB activity. Tax and TAB2 cooperatively activate TAK1 when they are coexpressed. Furthermore, TAK1 activation by Tax requires TAB2 binding as well as ubiquitination of Tax. We also found that the overexpression of TRAF2, 5, or 6 strongly induces Tax ubiquitination. These results suggest that TAB2 may be critically involved in Tax-mediated activation of TAK1 and that NF-κB-activating TRAF family proteins are potential cellular E3 ubiquitin ligases toward Tax.

Optineurin Negatively Regulates TNFα- Induced NF-κB Activation by Competing with NEMO for Ubiquitinated RIP

NF-kappaB essential modulator (NEMO), the regulatory subunit of the IkappaB kinase (IKK) that activates NF-kappaB, is essential for NF-kappaB activation. NEMO was recently found to contain a region that preferentially binds Lys (K)63-linked but not K48-linked polyubiquitin (polyUb) chains, and the ability of NEMO to bind to K63-linked polyUb RIP (receptor-interacting protein) is necessary for efficient tumor necrosis factor alpha (TNFalpha)-induced NF-kappaB activation. Optineurin is a homolog of NEMO, and mutations in the optineurin gene are found in a subset of patients with glaucoma, a neurodegenerative disease involving the loss of retinal ganglion cells. Although optineurin shares considerable homology with NEMO, in resting cells, it is not present in the high-molecular-weight complex containing IKKalpha and IKKbeta, and optineurin cannot substitute for NEMO in lipopolysaccharide (LPS)-induced NF-kappaB activation. On the other hand, the overexpression of optineurin blocks the protective effect of E3-14.7K on cell death caused by the overexpression of TNFalpha receptor 1 (TNFR1). Here we show that optineurin has a K63-linked polyUb-binding region similar to that of NEMO, and like NEMO, it bound K63- but not K48-linked polyUb. Optineurin competitively antagonized NEMO's binding to polyUb RIP, and its overexpression inhibited TNFalpha-induced NF-kappaB activation. This competition occurs at physiologic protein levels because microRNA silencing of optineurin resulted in markedly enhanced TNFalpha-induced NF-kappaB activity. These results reveal a physiologic role for optineurin in dampening TNFalpha signaling, and this role might provide an explanation for its association with glaucoma.

Tumor Necrosis Factor Receptor 2 Signaling Induces Selective c-IAP1-dependent ASK1 Ubiquitination and Terminates Mitogen-activated Protein Kinase Signaling

TRAF2 and ASK1 play essential roles in tumor necrosis factor alpha (TNF-alpha)-induced mitogen-activated protein kinase signaling. Stimulation through TNF receptor 2 (TNFR2) leads to TRAF2 ubiquitination and subsequent proteasomal degradation. Here we show that TNFR2 signaling also leads to selective ASK1 ubiquitination and degradation in proteasomes. c-IAP1 was identified as the ubiquitin protein ligase for ASK1 ubiquitination, and studies with primary B cells from c-IAP1 knock-out animals revealed that c-IAP1 is required for TNFR2-induced TRAF2 and ASK1 degradation. Moreover, in the absence of c-IAP1 TNFR2-mediated p38 and JNK activation was prolonged. Thus, the ubiquitin protein ligase activity of c-IAP1 is responsible for regulating the duration of TNF signaling in primary cells expressing TNFR2.

The Modern Art of Atherosclerosis

Atherosclerosis is a disease of large arteries that accounts for more than 50% of all deaths in the developed countries1 and, according to the World Health Organization prediction, it will gain a status of the major morbidity cause worldwide by the year 2010.2 Atherosclerosis is widely viewed as an inflammatory disease with hypercholesterolemia being a dominant underlying risk factor.3,4 It is believed to be initiated by retention of LDL particles in the lesion-prone areas, which is followed by monocyte recruitment and their differentiation into cholesterol-laden macrophage foam cells.1,5 Excessive cholesterol accumulation in macrophages exaggerates innate immune response that is manifested by upregulated production and secretion of inflammatory cytokines and chemokines, thus dramatically amplifying initial signal originated from the injured artery.6 See page 519 Despite the overall consensus on the causative roles of excessive cholesterol build-up and inflammation in the development and progression of atherosclerosis, the relationship between aberrant cholesterol metabolism and exaggerated innate immune response has not been totally established. Recent reports showing reduced inflammation in association with the cholesterol-lowering statin therapy provided indirect evidence linking cholesterol metabolism with regulation of immune response.7 However, this class of cholesterol lowering drugs not only inhibits cholesterol biosynthesis but also suppresses the synthesis of isoprenoids such as farnesyl pyrophosphate and geranylgeranyl pyrophosphate. Thus, inhibition of prenylation pathways such as those required for activation of Ras and Rho GTPase family proteins may be responsible for some of the …

Specificity of TRAF3 in Its Negative Regulation of the Noncanonical NF-κB Pathway

Tumor necrosis factor (TNF) receptor-associated factors (TRAFs) are critical signaling adaptors downstream of many receptors in the TNF receptor and interleukin-1 receptor/Toll-like receptor superfamilies. Whereas TRAF2, 5, and 6 are activators of the canonical NF-kappaB signaling pathway, TRAF3 is an inhibitor of the noncanonical NF-kappaB pathway. The contribution of the different domains in TRAFs to their respective functions remains unclear. To elucidate the structural and functional specificities of TRAF3, we reconstituted TRAF3-deficient cells with a series of TRAF3 mutants and assessed their abilities to restore TRAF3-mediated inhibition of the noncanonical NF-kappaB pathway as measured by NF-kappaB-inducing kinase (NIK) protein levels and processing of p100 to p52. We found that a structurally intact RING finger domain of TRAF3 is required for inhibition of the noncanonical NF-kappaB pathway. In addition, the three N-terminal domains, but not the C-terminal TRAF domain, of the highly homologous TRAF5 can functionally replace the corresponding domains of TRAF3 in suppression of the noncanonical NF-kappaB pathway. This functional specificity correlates with the specific binding of TRAF3, but not TRAF5, to the previously reported TRAF3 binding motif in NIK. Our studies suggest that both the RING finger domain activity and the specific binding of the TRAF domain to NIK are two critical components of TRAF3 suppression of NIK protein levels and the processing of p100 to p52.